x86: Add a 64-bit 'coreboot64' build

Coreboot is a first-stage bootloader mostly used on x86 devices as an
alternative to UEFI. Coreboot runs in 32-bit mode.

U-Boot currently supports booting from coreboot as a second-stage
bootloader, also in 32-bit mode. However it is useful to be able to run
U-Boot in 64-bit mode. To do this we can have a 32-bit SPL which switches
over the CPU and jumps to a 64-bit U-Boot proper.

Add a new 'coreboot64' board for running 64-bit U-Boot from coreboot. This
uses binman to create an image with a 32-bit SPL and a 64-bit U-Boot.

This allows running 64-bit EFI images on x86, for example, without needing
a native U-Boot port for a board.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>